Generally, methods to manufacture a metal pipe include manufacturing methods for electric resistance welded pipes and spiral tubes in which a pipe shape is formed by welding a metal strip while bending, and manufacturing methods for seamless tubes in which a hole is directly opened in a metal billet, and manufacturing methods in which a tube is formed by extrusion.
Electric resistance welded pipes have particularly high productivity, and are mass producible due to being producible at low lost. With such electric resistance welded pipes, an open pipe is formed in a circular tube shape from a running metal strip, and then a high frequency current is made to flow in end parts of the open pipe that face each other across an opening (also referred to below simply as “end parts of the open pipe”) and, in a state of being heated to melting temperature, the two end faces of the two end parts of the open pipe are pressed together into a pipe shape using rolls and welded. When this is being performed, as a method to supply current to the end parts of the open pipe, one example of a method is to wind an induction coil (solenoid coil) so as to surround the outer circumference of an open pipe, and to directly generate an induction current in the open pipe by causing a primary current to flow in an induction coil (see, for example, Patent Document 1 and Non-Patent Document 1), and another method is to press metal electrodes against the end parts of the open pipe, and to directly electrify using current from a power source. A high frequency current of from approximately 100 kHz to approximately 400 kHz is generally employed as the current passing through the induction coil or the electrodes at this time, and a ferromagnet known as an impeder is often placed at the inner face side of the pipe. The impeder is employed to prevent induction current that does not contribute to welding, due to attempting to circulate around the internal circumference of the open pipe.
As a method for causing an induction current in an open pipe, as described in Patent Document 2, there is what is referred to as a toroidal field (TF) heating method in which an induction heating coil with iron core is disposed above the end parts of an open pipe, and the end parts are heated by the action of an alternating magnetic field generated in an iron core by the flow of current in the induction heating coil. However, in a TF method, when the frequency of the supplied current is raised while attempting to reach the melting temperature, since only the outer surface of the welding piece is melted, resulting in defective melting, TF methods are merely employed for the manufacture of electric resistance welded pipes at the pre-heating stage with a low frequency current of from approximately 1 to approximately 3 kHz, as in Patent Document 2.